The present invention relates to an inlet system for an internal combustion engine.
As is well known to those skilled in the art, it is during operation within a relatively narrow engine speed range, which range is determined by a flow cross sectional area and a length of an inlet passageway, that the admission of intake air by suction to each of cylinders of an engine is effective.
According to a prior art inlet system disclosed in Japanese Utility Model Provisional Publication No. 56-105626, there are arranged between a surge tank and a cylinder head 22 a main inlet passageway 23 and an auxiliary inlet passageway 24, as shown in FIG. 1, in order to charge each of cylinders of an engine. The flow cross sectional area and the length of the main inlet passageway 23 are adjusted to optimize operation of the engine at a relatively high engine speeds, and those of the auxiliary inlet passageway 24 are adjusted to optimize operation at relatively low engine speeds. A control valve 25 is provided in the main inlet passageway 23, which valve is closed or opened to admit intake air through the main intlet passageway 23 or through the auxiliary inlet passageway 24 in order to improve the intake efficiency over a wide operating range of the engine.
However, according to this prior art inlet system, since the auxiliary inlet passageway 24 joins with the mating main inlet passageway 23 at generally right angles, the flow resistance at the junction is large. This makes it difficult to attain sufficiently increased power output during operation at low engine speeds where the auxiliary inlet passageway 24 only is put into operation. Another drawback with this prior art inlet system is in that it is difficult to form each of the auxiliary inlet passageways sufficiently long because of the fact that each of the main inlet passageways 23 and the mating one of the auxiliary inlet passageways 24 are formed as an integral unit by casting.